This invention relates to a method for determining the value of the MTF (modulation transfer function) of a viewing window, such as the viewing window used for or in a television or a data display tube.
There are many situations where a picture or pattern on the back surface of a viewing window is viewed through the window at some distance from the front or viewing surface. Two examples of such situations are windows used in color television and information data displays, both of which may be parts of cathode-ray tubes. In both cases, the front surface may be untreated or may be treated, as by grinding, etching, coating or other process, to reduce reflection or glare from ambient light sources. Such treatment may result in a reduction in the resolution or viewability of pictures or patterns that are displayed on the back surface of the window.
Until now, it has been the practice to measure the specular reflectance or the roughness of the front surface, and then to evaluate subjectively the relative sharpness or blurriness of a picture on the back surface of the window as viewed through the window. Quantitative comparisons of the viewability quality of the window are not possible with such subjective evaluations.
MTF (modulation transfer function) has been used to characterize the resolution of details of a picture or pattern for various components of a television system and in a conventional chemical photographic process. See, for example, "A New System of Measuring and Specifying Image Definitions," O. H. Schade, National Bureau of Standards Circular No. 526, Apr. 29, 1954, and "Image Quality--A Comparison of Photographic and Television Systems," O. H. Schade, Sr., RCA Laboratories, Princeton, N.J., 1975. However, such publications do not provide a method for determining the value of the MTF of a viewing window, particularly a method that can be used for routinely grading viewing windows both before and after the window is incorporated into an operable device.